1. Field of the Invention
The invention relates generally to direct-current machines having stator permanent magnets.
2. Description of the Prior Art
Direct-current machines of the type to which the invention relates are disclosed, for example, in German patent specification DE-OS 25 58 958. In those prior art machines, V-shaped bevels at the ends of the annular permanent-magnet shells in the stator frame are employed to reduce the excitation flux area toward the pole ends, so that during rotation of the rotor, the magnetic field does not break off abruptly but passes continuously into the neutral zones between the poles.
If the bevels of the annular permanent magnets, arranged one behind the other in the circumferential direction, are of a suitable design, the detent torque and, thus, the variations in permeance are reduced as compared with annular permanent magnets having no such bevels, but the fabrication of such bevels at the ends of the annular permanent magnets and the mounting of such shells in the stator frame are very costly manufacturing steps.
The manner in which the invention provides the desired detent torque reduction in a much less expensive manner will be evident as this description proceeds.